You have connected all your neurons, but you still need billions of "brain glue" cells--the neuroglia, which outnumber neurons by around 10 to 1. In recent years scientists have begun to recognize the importance of these cells, especially the enigmatic ones called astrocytes.

By inducing cells that line the capillaries to keep a tight seal, astrocytes maintain the blood-brain barrier, which protects the brain from many circulating molecules.

Astrocytes also form their own long-distance communication networks by "talking" via waves of calcium ions, and, like neurons, they can receive and release neurotransmitters.

Astrocytes in the human cerebral cortex are so big and complex, compared with equivalent cells in other mammals, that some researchers think they may hold a key to our cleverness.